Tropical cyclone Irina is expected to hit the northern coast of KwaZulu-Natal over the next two to three days, the SA Weather Services said on Friday.
The storm system will dump an "extreme" amount of rain over the region, it warned.
"Areas around Kosi Bay, St Lucia, and Richard's Bay can expect up to 400 millimetres of rain over this period," weather spokeswoman Hannelee Doubell said.
The amount of rain expected was of an "extreme nature" last experienced when tropical cyclone Domoina hit KwaZulu-Natal in 1984.
"Domoina brought between 400 and 600 millimetres of rain over a period of two to three days," Doubell said.
She said the system should show signs of weakening by Tuesday.